Standards for variable data printing (VDP) have come a long way since the first work by CGATS to develop a universal delivery format in the late 1990s. In 2010 the International Standards Organization published the PDF/VT standard, marking the first really effective specification for a reliable, vendor-neutral exchange of variable data jobs, both within and between companies.
A special type of the PDF file format, PDF/VT is specifically used for variable data and transactional printing in a variety of environments, from desktop printing to high volume digital production presses. Built on PDF/X, it therefore brings all the advantages of that standard in enforcing best practices for reproducible and predictable color and appearance to the variable data and transactional print worlds.
The industry is gradually realizing its value to improve quality, competitiveness and productivity, and I’ve been working with the PDF/VT Competence Center, especially with Christoph Oeters (Sofha), Paul Jones (Teclyn bv) and Tim Donahue (technical consultant) to produce a new set of Application Notes highlighting the benefits of using PDF/VT and the workflows that it enables.
The Application Notes explain how to make the highest quality and most efficient PDF/VT files to achieve the required visual appearance of a job, so if you develop software to read and write PDF/VT files, for example in composition tools, RIPs, digital front ends and imposition tools, or if you work on print workflow integration, you’ll find the notes really beneficial. They also show how document part metadata can be applied and leveraged for VDP specific production workflows.
Of course, there are wider benefits to using PDF/VT: The adoption of PDF/VT will allow the industry to finally move towards a reliable, vendor-neutral exchange of variable data jobs, simplifying the process of variable data printing significantly.
There’s been a lot of emphasis in the industry recently on perceived resolution. I’m sure you will have come across the phrase from major vendors:
“The Xerox Rialto 900 (…) offers 1,000 dpi perceived resolution for high quality output.”
Oce Vaior Print i300: “The multilevel dot modulation in combination with 600x600dpi resolution boosts the print quality of image elements and shadings to perceived 1200 dpi.”
But what is resolution anyway, and is it the only thing we need to worry about to ensure high quality output?
How we perceive resolution has changed over the years. For conventional print and first generation digital presses (except for wide format), resolution was two dimensional (across and along the media). More recently, inkjet presses (and some toner) can place different amounts of colorant at each location on the substrate, using greyscale heads, multiple passes with the same head, or multiple heads imaging at the same location. This means that resolution has effectively become 3D: not only along and across the media, but also in the amount of colorant applied at any single pixel position.
At Global Graphics we call this “multi-level output”, compared to the “binary” output where each pixel can either be coloured or not, with no intermediate steps.
Resolution? Or addressability and droplet size? As print geeks know well, press resolution has very little to do with resolving power, it is really a marketing simplification to use the word ‘resolution’ for ‘addressability’ – e.g. at 600 dpi, each addressable pixel is 1/600” from its neighbours. The detail that can be displayed is a factor of droplet size as well as addressability; as droplets get bigger each one covers more than just a single (square!) pixel on the media, so less fine detail is retained.
Droplet placement accuracy also comes into play. In a perfect world we would have a regular grid of droplets, but in practice we don’t usually get one. The variation in separation between droplets can lead to coalescing, mottling or streaking on some substrates, especially on UV inkjet presses, but it can occur on aqueous as well.
Addressability and droplet size affect the rendering of small type and other high-contrast fine detail. Droplet placement accuracy affects texture of final print. So we still don’t have a clear metric for “perceived resolution” …
What about resolution and bit depth? Using multi-level output can produce smoother rendering of images and other graphics with gradual tone or colour changes than binary output at the same resolution can achieve.
But nozzle redundancy is also vital: In a single pass press, with a page-wide array, a single blocked nozzle will leave a white line down the substrate unless something is built in to fix that, such as nozzle redundancy. And that redundancy must use up some of the press’ capability to use multiple nozzles in the same location for multi-level output, so 1200 dpi nozzles often doesn’t mean 1200 dpi addressability on the substrate.
And sometimes each nozzle can only deliver one droplet size; sometimes it can deliver a variety of sizes.
So what’s the real quality that these presses are capable of? We need a lot of information to really understand what’s going on: dpi across and along the media, number of nozzles imaging any single pixel, droplet sizes available from that nozzle, proportion of nozzles used for redundancy … I don’t think I’ve ever seen a press vendor’s public specification that gives us all the information we want.
Can we even say, simplistically, that higher resolution and bit depth are good? If everything else is equal then yes, in many cases, except that you can push either too far. On an aqueous inkjet, higher resolutions really need smaller highlight droplets; smaller lone droplets tend to disappear into some media and can lead to loss of extreme highlights on the output. Interestingly you end up with output that looks remarkably close to the way flexo loses those same highlights!
And you also need to remember that higher addressability means high computational requirements, and more computations mean more expensive DFEs, higher running costs, maybe even less green … (a faster RIP can offset this, of course!) It also makes the press more expensive, and harder to run as fast.
And what’s the impact on quality? There are other factors other than bit depth, addressability and droplet size and placement which affect the final result, for example:
Items affecting ink spread or movement on the substrate such as paper smoothness, absorbency, coatings, ink viscosity and surface tension;
Movement of the colorant into the substrate, reducing the capability of showing very small detail or saturated colours.
Colour management, including ink limitation and reduction
So the ‘virtual’, mathematical discussion of resolution and droplet size are is certainly not the only factor in determining the quality of output. Quality arises from a complex mix of heads, electronics, wave forms, inks, media, resolution, registration, bit depth and half-toning etc. We don’t have a good way to provide a single, understandable quality metric to sum it all up. ISO DTS 15311-1 is defining testing and reporting methodologies in this area, although it still doesn’t provide a simple quality metric.
So what’s the answer? We just don’t have a single number that sums up the quality capability of a digital press at the moment. But then simply reporting ‘resolution’ has never really fulfilled that role in the past for binary systems, from imagesetters to platesetters to office printers … to digital cameras. So perhaps we shouldn’t be too disappointed.
What should you do when a vendor reports “perceived resolution”? I’d suggest that you take it as an indication of the level in the marketplace that the vendor is intending to address … and then draw your own conclusions based on print samples.
If you’re looking to buy a press, have the vendor:
Print samples on the media and at the speed that you expect to use
Use a variety of graphical constructs to explore press behaviour:
Flat tints at a range of tones and colours
Smooth graduations, including some long ones all the way to white
Photographic images, including high and low key, soft-focus and sharp detail
Fine vector detail such as small serif and sans serif text
If you’re already running a press do the same. Each technology has different strengths and weaknesses; you may even need multiple presses to address all work in your particular target sector. The key thing is to understand what your presses are good at, and what to avoid, and then to work with your customers to achieve the best possible result … and to set expectations appropriately in advance.
If you’re a press vendor, talk to us about how Global Graphics’ multi-level screening technologies can maximise the quality and the value of your hardware.
Readabout our latest advances in screening, presented at the Inkjet Conference, October 2015.
Making progress in half-tone screening technology – our samples are ready to display!
We’re really looking forward to the Inkjet Conference in Düsseldorf next week. Global Graphics’ CTO, Martin Bailey, will be speaking at the conference and focusing on the problems inkjet vendors have encountered when printing on high-speed inkjets, particularly with regard to optimum image quality and droplet placement.
With this in mind, for the last few months we’ve been working with a number of inkjet press manufacturers to develop entirely new half-tone screening technology for presses that can vary the amount of ink delivered in any one location on the media. We’ve just received our sample prints to show you at the Conference and we’re really pleased with the results – you can see the improvement immediately.
The samples show typical ‘before and after’ scenarios: The ‘before’ samples are quite noisy and show mottle and puddling; the ‘after’ samples, printed with Global Graphics screening technology, show much smoother gradients where we manage the transition of droplet size in multi-level heads.
We have also prepared sample prints showing what the output looks like with no tuning on: They show noise and steps in gradients for multi-level output, then we demonstrate what happens when we use transition points of drop size when using inks such as white, orange and violet in the colour spectrum.
Look out for Martin at the Conference and drop by our table in the IJC Networking Arena to see the prints for yourself.
If you are interested in the benefits of half-tone screening on high-speed inkjets and would like to join our research programme, watch our video here for more information: https://www.youtube.com/watch?v=WNrSbb46efg.
Our Harlequin product team has launched a set of hybrid screens specially developed to give premium quality in flexo work.
The screens address the well-known issues of how to achieve high-quality in the highlight areas of images, such as tones close to white or skin tones, and how to print those areas with smooth gradations.
“We used the Harlequin Cross Modulated™ screens as the basis for development and have expanded the number of screens and included a mechanism to auto select calibration that goes with a particular screen,” comments Martin Bailey, CTO, Global Graphics.
“With the new Harlequin Cross Modulated Flexo (HXMFlexo) screens you can produce high-quality graphical objects by selecting from a wide choice of screen resolutions, rulings and dot sizes. Pre-press operators also now have the ability to bump up curves at the highlight end to compensate for flexo not being able to produce tones close to white clearly, so you can achieve smooth gradations even in high-key images.”
The new screens are the result of working with our OEM partners in the flexo market who have used the Harlequin RIP for years and we’ve been able to take input from a variety of vendors to fine tune our plans.
HXMFlexo works with the latest editions of the Harlequin RIP.
For those who missed it at PODi and XPlor, I’m presenting two free webinars this week on “Do PDF/VT Right: how to make problem free files for variable data printing”, hosted by Dscoop University – the Digital Solutions Cooperative. You don’t have to be a member of Dscoop, nor an HP Indigo user to join.
If you’d like to sign up to hear me talk through the guide, download the “Top 5 tips” poster, and have the chance to ask any questions, please register using the following links:
I’m really excited to be involved with rolling out the Harlequin Partner Network. The Harlequin team in Global Graphics has worked with a variety of other companies in the prepress and digital print ecosystem for many years:
We’ve reached out to file creators when we’ve found non-compliant or inefficient constructs in PDFs and other files.
We’ve worked with other vendors to develop closer integration of workflow components with Harlequin, such as using the push calibration API introduced in Harlequin Server RIP 9.
We’ve collaborated to approaches from other players in the ecosystem round Harlequin who have identified new products or approaches to improve workflows for users.
But those relationships have always been rather informal and often transitory. The Harlequin Partner Network takes that to the next level, giving us a proper framework to work together on closer and more efficient integration of a variety of products from multiple vendors. And it also gives us better ways to tell the world about what we and those partners are doing.
I see it as a win for Global Graphics, allowing us to identify and work more closely with vendors of products that are used together with Harlequin.
It’s a win for our new Partners, allowing them better access to technical and marketing resources around Harlequin.
And it’s a win for our users, because our closer engagement with other vendors will lead to closer collaboration and therefore to more efficient integrations that will benefit print shops in the future.
We know we haven’t yet identified all the products and vendors who can help build this community. If you think you should be involved, please let me know!